Electrolysis and extracting metals
Chemistry · GCSE · The School
Splitting with electricity
ELECTROLYSIS uses an electric current to break a compound into its elements. The compound must be molten or dissolved so its ions can move. Positive ions travel to the negative electrode (the CATHODE) and gain electrons; negative ions travel to the positive electrode (the ANODE) and lose them.
What forms where
At the cathode, metal forms — unless the metal is more reactive than hydrogen and the solution is aqueous, in which case hydrogen is produced instead. At the anode, a halogen forms if one is present; otherwise oxygen. Working out the products means asking what ions are present and which is discharged more readily.
Why the method depends on reactivity
Metals LESS reactive than carbon — iron, zinc, copper — can be extracted by heating their ore with carbon, which is cheaper. Metals MORE reactive than carbon — aluminium, sodium, magnesium — cannot, because carbon will not displace them, so electrolysis is required. That is why aluminium remained rare and expensive until electrolysis was industrialised.
Iron sits BELOW carbon in the reactivity series, so carbon can take the oxygen from it: iron oxide + carbon → iron + carbon dioxide, in a blast furnace, and cheaply. Aluminium sits ABOVE carbon, so carbon cannot reduce it — the aluminium holds its oxygen more strongly. It must be electrolysed, which is why aluminium was once more precious than gold and is still made where electricity is cheapest.